In the past several proposal have been made for obtaining an imaging element that can be developed using only dry development steps (hereinafter called dry imaging element) without the need of processing liquids as for example in silver halide photographic materials. Two major classes of dry imaging elements can be distinguished by the method used for exposing them.
According to one class an image is formed in a photosensitive layer using exposure to actinic radiation. Generally the photosensitive layer comprises a photopolymerizable composition. The use of photopolymerizable compositions for the production of images by information-wise exposure thereof to actinic radiation is well known. Several methods for forming images using photopolymerizable compositions are known. All these methods are based on the principle of introducing a differentiation in properties between the exposed and non-exposed parts of the photopolymerizable composition e.g. a difference in solubility, adhesion, conductivity, refractive index, tackiness, permeability, diffusibility of incorporated substances e.g. dyes etc. The thus produced differences may be subsequently employed in a developing step to produce a visible image and/or master for printing e.g. a lithographic or electrostatic printing master.
The use of the difference in tackiness to obtain an image is described in e.g. U.S. Pat. Nos. 3,060,024, 3,085,488 and 3,649,268. According to the method disclosed in these U.S. patents the image-wise exposed photopolymerizable composition looses its tackiness in the exposed parts while the non-exposed parts keep their tackiness. The non-exposed parts can therefore be colored with dry dye pigments to make the image visible.
According to the methods described in e.g. U.S. Pat. No. 3,245,796 and EP-A 362,827 the diffusibility of a dye is impeded in the photo-exposed parts of the photopolymerizable composition so that during an overall thermal heating subsequent to photo-exposure dye substances in the non-exposed areas will be able to diffuse to a receptor material. According to a similar method described in U.S. Pat. No. 4,587,198 the photopolymerizable composition is made impermeable in the exposed parts for a sublimable dye or dye-pigment present in a layer contiguous to the layer comprising the photopolymerizable composition.
According to a method disclosed in U.S. Pat. No. 3,060,023 the adhesion of the photopolymerizable composition is modified upon image-wise exposure. After image-wise exposure the non-exposed parts will stick or adhere, during a step of overall heating, to a receiving sheet thus allowing the transfer of the non-exposed parts to the receiving sheet.
On the other hand dry imaging elements are known that can be image-wise exposed using an image-wise distribution of heat. These types of dry imaging elements also called heat recording materials offer the advantage in addition to an ecological advantage that they do not need to be handled in a dark room nor any other protection from ambient light is needed. A disadvantage of heat mode recording materials is their low sensitivity requiring powerful exposure means. This disadvantage is probably one of the major reasons why heat mode recording materials have not found wide acceptance up till recently despite their potential advantages. Since powerful exposure means especially lasers are becoming more readily available it may be expected that said disadvantage will no longer impair the wide spreading of heat mode recording materials.
Heat mode recording materials are disclosed in e.g. U.S. Pat. Nos. 4.123.309, 4.123.578, 4.157.412, 4.547.456 and PCT application WO 88/04237. The latter application discloses a web having an image forming surface and a porous layer of an image forming substance. The element further comprises a heat sensitive substance. Upon imaging with a laser the image forming surface is liquefied at the exposed parts thereby penetrating the porous layer and improving its adherence to the web while at the non-exposed parts liquefying of the image forming surface does not take place and as a consequence the adherence of the porous layer to the web remains poor. The porous layer can then be removed in the non-exposed areas using a stripping tape or by simply rubbing. Since the thus obtained image may be easily scratched and is very poor wear resistant it is necessary to laminate a protecting layer to the image which is inconvenient and makes such a heat recording material less attractive.
Furthermore due to the subtle balance of adhesion forces between the porous layer and the image forming surface and the cohesive forces within the porous layer the removal of the porous layer with a stripping tape has to be performed under very stringent conditions and even then lateral cracks of the porous layer in the exposed parts may occur resulting in a decreased image density.